Children are notoriously bad at delaying gratification to achieve later, greater rewards (e.g., Piaget, 1970)--and some are worse at waiting than others. Individual differences in the ability-to-wait have been attributed to self-control, in part because of evidence that long-delayers are more successful in later life (e.g., Shoda, Mischel, & Peake, 1990). Here we provide evidence that, in addition to self-control, children's wait-times are modulated by an implicit, rational decision-making process that considers environmental reliability. We tested children (M = 4;6, N = 28) using a classic paradigm--the marshmallow task (Mischel, 1974)--in an environment demonstrated to be either "unreliable" or "reliable". Children in the "reliable" condition waited significantly longer than those in the "unreliable" condition (p less than 0.0005), suggesting that children's wait-times reflected reasoned beliefs about whether waiting would ultimately pay off. Thus, wait-times on sustained delay-of-gratification tasks (e.g., the marshmallow task) may not only reflect differences in self-control abilities, but also beliefs about the stability of the world. (Contains 2 figures.)